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Well, Burt says he isn’t using microstepping, so he’s probably way under the limit, but that depends on Burt being in the right ballpark for steps/mm. What steps/mm are you using Burt? Are you starting with the 4500ish number from the stock machine?
I haven’t used grbl, but this page (for 0.9, not sure what version you have) https://github.com/grbl/grbl/wiki/Configuring-Grbl-v0.9 Shows that $20 is a boolean, and there are other limits (separated by axis, $110, $111, $112) for speed. Can you try with just adjusting the speed, and then driving it to a position manually to make your test easier?
Jason, Are you talking about the ramps FD with an arduino due? http://reprap.org/wiki/RAMPS-FD . You could definitely get that to work, but I don’t know if the screen works, and the marlin firmware doesn’t seem to support it natively, AFAIK (there is a marlin FD fork on github). I’ve used repetier firmware on my mega, and it seems to act very similar, so you could switch to that.
The easier thing to do is just remove the jumper on the Z axis, to go to 16 steps and reduce the steps/mm by 2 on just the Z axis. With 2k steps/mm instead of 4k steps/mm, I doubt there will be a noticeable difference on the Z.